This invention relates to a new process and apparatus for producing generally elongated composite articles of fiber-reinforced resins.
More specifically, this invention provides a novel process for continuously producing composite articles of cylindrical shape or other elongated shapes of fiber-reinforced thermosetting resins, which process, unlike the known processes, makes possible the high-speed, high-quality, economical production of such articles.
As a continuous process for producing composite articles of fiber-reinforced thermosetting resins, a so-called pultrusion process has heretofore been proposed. In such a pultrusion process, a fiber-reinforcing material in roving or mat state, or a combination thereof, is impregnated with the thermosetting resin, and the resulting composite is introduced into a heated die for shaping and setting, i.e., curing. The die typically has a passage defining the cross-section or shape of the ultimate product.
Accordingly, the conventional pultrusion processes involve drawing a bundle of filaments from a source thereof; wetting these filaments or reinforced fibers and impregnating them with a preferably thermosettable polymer resin by passing the filaments through a resin bath in an open tank; pulling the resin-wetted and impregnated bundle through a shaping die to align the fiber bundle and to manipulate it to the proper cross-sectional configuration; and curing the resin in a mold while maintaining tension on the filaments. Since the fibers progress completely through the pultrusion process without being cut or chopped, the resulting products have exceptionally high longitudinal strength. They are, therefore, uniquely suited for articles in which high longitudinal strength is required.
The use, in the prior art, of open resin tanks for impregnating the fibers or rovings continuously passing therethrough has the following grave disadvantages
1. In an open tank impregnator, there is the danger that the monomer or highly evaporative content in the resin compound will cause air pollution which, in turn, endangers the health and welfare of the workmen in the plant and can increase the danger of fires, etc.
2. The viscosity of the resin compound in the open resin tank changes with time due to monomer evaporation. This results in an uneven composition of the resin in the tank which, in turn, leads to unstable product quality and consequently a high rate of waste.
3. Mixing of the resin in the open tank with a hardener not only shortens the "shelf life" of the composition but also will cause gelling in the open impregnator tank. The residue which results must be frequently cleaned at substantial labor cost and downtime of the tank.
4. As a result of one or more of the foregoing disadvantages, it will be seen that the proportion of resin to fiber will not be consistent, due to the great difficulty in controlling the viscosity and the composition of the resin mixture in the open impregnation tank.
5. After leaving the open impregnation tank, the array of the fibers is often disordered, and the fibers are often twisted and damaged and their appearance is generally poor.
6. Products produced by conventional pultrusion techniques must frequently be reinforced by use of expensive filament material or fabric because of poor transverse directional performance characteristics.
7. The open tank impregnation method is not suitable for thermoplastic resins, particularly engineering plastic resins with a relatively high melting point.
In order to overcome the above-mentioned shortcomings, a great deal of attention has recently been given to improving the reinforcing materials, resins and hardening agents. For example, in reinforcing materials, quick wetting glass fibers and three-dimensional cross-linking reinforcing materials are now available in the marketplace. As to the hardeners, efforts have been made to accelerate hardening time and prolonging shelf life. Resins now in use have all of the advantages of low shrinkage, fire retardant quality, high temperature resistance, reduction of exotherm cracking and low volatility. Production facilities have been improved so that products of larger size can be produced, particularly by increasing the pull-force.
As far as the pultrusion technology is concerned, a number of new processes have been developed, including systems for permitting a rapid exchange of resin tanks, means for more uniformly heating the mold, systems for controlling more accurately the tension of reinforced fibers, and microcomputer process controls for many of the steps in the process. Unfortunately, all of the foregoing can solve only a portion of the existing problems. The problems created by the use of the open resin impregnating tank have not been solved.
The state of the art in continuous pultrusion processes is generally described in U.S. Pat. No. 4,394,338 to Masaru Fuwa and U.S. Pat. No. 4,445,957 to Dennis L. Harvey. Each of the foregoing patents relates to a continuous pultrusion process in which fiber-reinforcing material is impregnated with a thermosetting resin in an open resin impregnator tank. This traditional pultrusion technology has the shortcomings mentioned above.
In each of the aforesaid patents, the filaments are impregnated with a thermosetting polymer resin by being pulled through an open vessel filled with such resin. Then, the resin-impregnated bundle of filaments is pulled through a shaping die to align the filament bundles and to manipulate them into the proper cross-sectional configuration after which the composite is cured in a mold. It would be preferable in pultrusion processes of the type described in the aforesaid patents (a) to continuously mix specified quantities of resin and catalyst in desired proportion and only just prior to the use of such mixture for impregnation of the filaments, (b) to deliver only such quantities of mixture to the filaments as are required by the pulling rate of the filaments so that just the proper amount of mixture will be available for wetting and no more, and (c) to perform the steps of conveying the resin mixture to the filaments and wetting the latter out of contact with the atmosphere.
Accordingly, it is an object of the present invention to provide a method of forming an article by the pultrusion process which will overcome the disadvantages enumerated above.
Another object of the present invention is to provide a method of forming an article by the pultrusion process in which impregnation of the filaments takes place out of contact with the atmosphere.
A further object of the invention is to provide a method of forming an article by the pultrusion process in which the relative quantities of resin and catalyst can be accurately controlled in relation to the continuous quantities needed for continuous impregnation of the fibers.
Still another object of the invention is to provide a novel die which is capable of simultaneously impregnating and shaping reinforcing filaments to produce a continuous pultruded composite product of constant cross-sectional configuration.
A concomitant object of the invention is to provide a novel apparatus for impregnating and shaping continuous reinforcing fibers while maintaining the resin-impregnating material out of contact with the atmosphere.